Figure 5: a) Schematic illustration of water intake mechanism of dry NS and evaporation on the wet NS structure, Inset showing transportation of water via natural evaporation and gravity through the Micro/Nanochannels. b) Open Circuit Voltage (Voc) and Short Circuit Current density (Jsc) of four NSs. c) Long term reliability analyses of the WS-WEG for 1 week d) Typical Voc and Isc vs time for WS. e) Dry and wet WS as discharged and charged stage. f) Histogram of the Voc for 30 WS-WEG samples.
The resulting G-NS-G structure exhibits an open circuit voltage (Voc) above 500 mV and a short circuit current density (Jsc) of over 0.1 µA/mm2 with a small amount of 1 ml drop DI water. The maximum current density (Jsc) was achieved by the swift transportation of electrons along the complete electrical circuit. The streaming potential, explained later in the mechanism section, shows the positive polarity towards the flow directions of the DI water. Therefore, only the positive polarity is considered throughout this study. The Voc and Isc show stable performance for a couple of hours because of the constant evaporation of water into the air. The orientation of these micro/nano-channels is placed parallelly to the direction of gravity and capillary induced water flow.
Among these four NSs, the WS displayed the most efficient Jsc, exceeding 0.20 μA/mm2, and Voc, surpassing 600 mV. Multiple factors influence the output Voc and Jsc of these four NSs. Distribution of the micro/nanochannels on the shell structure, porosity and their surface charge are the key parameters that affect the output Voc and Jsc. Larger channel diameter, as seen in AS, shows low performance compared to WS, which can be explained by the equation of streaming potential. Because of the irregular channel structure and pore distributions, FS and PS also underperform compared with WS. Also, the asymmetrical-shaped narrow channels heighten hydrodynamic resistance. The highest evaporation rate of WS was observed in Figure 4 (c) also contributes to its superior electric performance.
WS exhibits superior uniformity, compact, and consistent channel structures among these four NSs, portrayed by large, interconnected 3D -puzzle cell, facilitating more efficient fluid flow than others.[28,30,31] This simple G-WS-G device can generate a stable Voc of 612 mV and Jscof 0.15 µA/mm2 for a long duration. It consistently maintained an open circuit voltage above 550 mV and short circuit current above 17 µA (with 12 mm X 12 mm) for over a week without any substantial fluctuations, as seen in Figure 5 (c). This signifies the long-term stability of the nutshell-based WEG devices. Replenishing the DI water every few hours at 0.3 ml/h maintained the evaporation process under 25% humidity and 25 ºC with full surface exposure. Because of the continuous evaporation of water, three layers are created on the shell structure: wet, dry, and partially wet. The partially wet regions present the highest contribution of streaming voltage.[49]
Figure 5 (d) demonstrates the progressive rise in Voc caused by the ongoing evaporation of DI water through the micro/nanochannels. The slower mobility of DI water leads to a prolonged duration for reaching the maximum voltage. A faster achievement of the stable state was feasible because of the simultaneous actions of gravity and capillary action. The Voc reaches its maximum level while the specific saturation level is reached, and it remains constant. Is decays because of the emission and readjustment of electrons and reaches a stable condition after a while. The NSs can continually harvest energy for a long time under wet conditions, however, the entirely dry NS doesn’t have the capability of generating energy. Therefore, the wetted NS can be considered as charged whereas the fully arid NS can be considered as discharged, that is depicted in Figure 5 (e). This nutshell-based WEG can be reused cyclically by drying the soaked NS and re-wetting it for energy harvesting.
Testing performed with platinum electrode (Figure-S 6 ) eliminates the possibilities of chemical reactions caused by the electrode contaminations. Output voltage and current values might vary slightly because of sample variations and measurement conditions. A total of 30 independent samples were inspected to confirm the reliability of the WS-WEG device (Figure 5 (f) ). The findings signify that most samples demonstrated the revealed Voc in the range of 580 mV to 620 mV.

Influential Factors Exploration on Device Performance

Multiple factors can influence the functionality of these WS-WEG devices during actual usage. Hence, a systematic investigation was carried out on several aspects, including relative humidity, temperature, different concentrations of NaCl solutions, heights of the shell structure, and polarity shifting of the device. The WS-WEG device was observed by placing it on different levels of water heights. It is observed inFigure 6 (a) that a partially submerged device, one end is exposed to air, sustains a consistent voltage due to water evaporation and the streaming potential effect. However, once the device becomes completely immersed, voltage reduces due to the inhibition of water evaporation, the major mechanism for electrical production. The water evaporation rate decreased across the micro/nanochannels for the fully submerged device since the channels of the fully submersed device were partially impeded. Capillary flow along the channels is less prominent in fully submerged devices. Therefore, these devices show lower voltage compared with the partially submerged ones. Upon removal from the water reservoir, the device maintained in generating a low amount of voltage for a limited duration due to the evaporation of residual water content until it was completely dried.